Stents are support structures that are implanted in tubular organs, blood vessels or other tubular body lumens to help keep such conduits open. Stents are often used following balloon angioplasty to prevent restenosis and may, more generally, be used in repairing any of a number of other tubular body lumens, such as those in the vascular, biliary, genitourinary, gastrointestinal, respiratory and other systems.
The materials used in fabricating stents must be chemically and biologically inert to living tissue. Stents must further be able to stay in position and continue to support the tubular body lumens into which they are implanted over extended periods of time. Moreover, stents must have the ability to expand from a contracted state, which facilitates insertion into a body lumen, to an expanded diameter that is useful in supporting at least a portion of the body lumen. This expansion is accomplished either mechanically, such as by the action of a balloon-ended catheter, or by self-expansion such as by shape-memory effects or by the use of a constrained elastic stent.
The above requirements limit the number of eligible stent materials. One of the most widely used metal alloy systems is the nickel-titanium system, the alloys of which are known as nitinol. Under certain conditions, nitinol is highly elastic such that it is able to undergo extensive deformation and yet return to its original shape. Elastic stents are typically deployed in a body lumen by reducing the diameter of the stent by mechanical means, restraining the stent in the reduced diameter during insertion into the body, and releasing the stent from the restraint at a target location. Once released, the stent “self-expands” to its predetermined, useful diameter by virtue of its elastic properties. One of the advantages of elastic stents is that, after deployment, they are able to “rebound” to their useful diameters after being deformed by external forces. The resilient nature of such stents not only make them ideal for self-expansion upon delivery to a target location, but it also makes them desirable for use in body lumens that are often subjected to external forces and corresponding temporary reductions in diameter or other deformations. For example, elastic stents are useful for placement in the carotid artery, which is frequently deformed by external forces because the vessel is in close proximity to the body surface.
There are, however, some potential drawbacks associated with conventional elastic stents. For example, such self-expanding stents possess a single predetermined diameter, thus limiting the use of a given stent and increasing the number of different stents required to cover a range of useful diameters. Where the predetermined diameter is larger than the body lumen in which the stent is placed, residual expansion forces often result in the gradual, undesired expansion of the surrounding lumen. The release of self-expanding stents often does not exert sufficient force to open blocked body lumens containing hard plaque. It is thus sometimes necessary to perform the additional step of inserting a balloon into the partially-deployed stent for further dilation, thus adding cost, time and risk to the overall procedure.